In a conventional suspension system, a road wheel is mounted for movement upwardly and downwardly relative to the body. A coil spring and a shock absorber are typically arranged between the body and the supporting structure on which the wheel is mounted. The shock absorber controls the flow of fluid between two chambers, and functions to dampen the velocity of relative movement between the body and the road wheel.
In recent years, there has been considerable interest in developing an "active" vehicle suspension system. See, e.g.: "Lotus' active suspension", Automotive Engineer (Febrary/March 1984) [pp. 56-57]; McCosh, "no-springs, no-shocks suspension", Popular Science (July 1986) [pp. 60-63]; and McCosh, "Springless Corvettes", Popular Science (Sept. 1986) [p.12]. In these "active" systems, the conventional spring and shock absorber are replaced by a servocontrolled double-acting hydraulic actuator. Suitable sensors, such as accelerometers and the like, are used to sense and determine certain parameters, such as body attitude and acceleration. The signals generated by these sensors are appropriately tailored and used as command signals to servovalves, which control the flow of fluid to the actuators. Thus, rather than merely responding passively and reactively to various forces acting on the vehicle, the "active" systems may be used to affirmatively correct and compensate for the effects of such forces. For example, during braking, the conventional vehicle will "nose down". However, with an actively-controlled system, the various servos may be operated, either independently or in combination with one another, to maintain the body level and horizontal during braking, acceleration and cornering.
In some hybrid or semi-"active" systems, the conventional spring remains, but the shock absorber is replaced by a servocontrolled actuator. Such spring and actuator may be arranged either in parallel or in series with one another.
In any event, in such "active" and "semi-active" systems, and in still other level- and attitude-control systems (e.g., those employing variable-rate shock absorbers having fluid damping characteristics which vary with position), it is often desired to known the relative positions between the wheel and body. Such data may, for example, be used as a feedback signal in a closed-loop servocontrol system. There are many devices available for sensing position and converting it into an analog electrical signal. One such device is a linear variable differential transformer (LVDT), which has a core movable relative to a sensing coil. Such devices could, of course, be connected directrly to the body and the wheel. However, the typical stroke of relative movement between the wheel and the body is relatively long, and this would require the use of a correspondingly long-stroke LVDT.